Distribution cutouts are widely used for pole-top mounting and protection of distribution circuits. These fuse cutouts must be low in cost, as well as possessing relatively high interruption ratings. As distribution circuits continue to grow in size, the demand is inherent for higher-interruption capacity fuses.
An obvious candidate to fill this need for higher-capacity distribution cutouts is the current-limiting fuse. However, there are numerous design difficulties in translating the idea of a current-limiting fuse into a distribution cutout concept. One of these problems is to provide a pigtail release-type of a connector at one end of the fuse. Mechanical release is effected at the time of "blowing" of the fuse. Another significant problem concerns the interruption of low currents. Although current-limiting fuses are built to successfully interrupt low currents, this low-current interruption is always characterized by a relatively long arcing time, until final clearing is effected. This phenomena conflicts with the necessity of having a drop-out type of action, such as distribution fuse cutouts must possess. The long arcing times indicate a significant time delay between the release of the pigtail (drop-out initiator) and the final clearing of the fault. This results in the fuse drop-out action being effected before the final clearing, and thus, the likelihood of external arcing between the moving fuse ferrule and the external cutout contacts. This is an untenable situation, as such arcing could result in external flashover of the fuse mounting.
A further problem is one of economics. The current-limiting fuse must be basically a more expensive type of device than is the fiber-lined tube of a distribution cutout. Even though the higher capacity inherent in the current-limiting fuse may not always be needed -- dependent upon the type of fault and its location -- the more expensive fuse would need to be provided to furnish the requisite capacity for a possible fault right at the terminals of the device.
U.S. Pat. No. 3,467,934, issued Sept. 16, 1969, to Robert T. Innis and George E. Mercier, teaches a two-part indicating fuse adaptable for capacitor-unit protection, and including a flexible fuse casing. It is contemplated in this patent that when fuse operation occurs, there will result a breakaway action between the two fuse casing sections of the device to thereby permit a visible indication of fuse operation.